Generation of High-Density High-Polarization Positrons via Single-Shot Strong Laser-Foil Interaction

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Generation of High-Density High-Polarization Positrons via Single-Shot Strong Laser-Foil Interaction

Kun Xue, Ting Sun, Ke-Jia Wei, Zhong-Peng Li, Qian Zhao, Feng Wan, Chong Lv, Yong-Tao Zhao, Zhong-Feng Xu, and Jian-Xing Li

Phys. Rev. Lett. 131, 175101 – Published 24 October 2023

Abstract

We put forward a novel method for producing ultrarelativistic high-density high-polarization positrons through a single-shot interaction of a strong laser with a tilted solid foil. In our method, the driving laser ionizes the target, and the emitted electrons are accelerated and subsequently generate abundant $γ$ photons via the nonlinear Compton scattering, dominated by the laser. These $γ$ photons then generate polarized positrons via the nonlinear Breit-Wheeler process, dominated by a strong self-generated quasistatic magnetic field $B^{S}$ . We find that placing the foil at an appropriate angle can result in a directional orientation of $B^{S}$ , thereby polarizing positrons. Manipulating the laser polarization direction can control the angle between the $γ$ photon polarization and $B^{S}$ , significantly enhancing the positron polarization degree. Our spin-resolved quantum electrodynamics particle-in-cell simulations demonstrate that employing a laser with a peak intensity of about $10^{23} W / {cm}^{2}$ can obtain dense ( $≳ 10^{18} {cm}^{- 3}$ ) polarized positrons with an average polarization degree of about 70% and a yield of above 0.1 nC per shot. Moreover, our method is feasible using currently available or upcoming laser facilities and robust with respect to the laser and target parameters. Such high-density high-polarization positrons hold great significance in laboratory astrophysics, high-energy physics, and new physics beyond the standard model.

Received 7 June 2023
Accepted 19 September 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.175101

Physics Subject Headings (PhySH)

Beam polarization Compton scattering Gamma-ray generation in plasmas High intensity laser-plasma interactions High-energy-density plasmas Laser driven electron acceleration Laser-induced magnetic fields in plasmas Magnetic field generation & plasma dynamo Particle acceleration in plasmas Polarization of light Quantum description of light-matter interaction Radiation & particle generation in plasmas

Accelerators & BeamsAtomic, Molecular & OpticalPlasma Physics

Authors & Affiliations

Kun Xue ¹, Ting Sun¹, Ke-Jia Wei¹, Zhong-Peng Li ¹, Qian Zhao¹, Feng Wan^1,*, Chong Lv^2,†, Yong-Tao Zhao¹, Zhong-Feng Xu¹, and Jian-Xing Li ^1,2,‡

¹Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter (MOE), Shaanxi Province Key Laboratory of Quantum Information and Quantum Optoelectronic Devices, School of Physics, Xi’an Jiaotong University, Xi’an 710049, China
²Department of Nuclear Physics, China Institute of Atomic Energy, P.O. Box 275(7), Beijing 102413, China

^*wanfeng@xjtu.edu.cn
^†lvchong@ciae.ac.cn
^‡jianxing@xjtu.edu.cn

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Vol. 131, Iss. 17 — 27 October 2023

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Images

Figure 1
(a) Interaction scenario. The laser propagates along $+ \hat{x}$ and is polarized along $P_{L}$ , with the laser polarization angle $ϕ_{L}$ between $P_{L}$ and $+ \hat{y}$ ; the foil has a tilt angle $θ_{T}$ with respect to the $\hat{x} − \hat{z}$ plane. (b) Direct laser acceleration (DLA). The brown, red, and blue arrows represent the directional quasistatic magnetic field $B^{S}$ , the surface electric current $J_{f}$ , and the return current $J_{r}$ , respectively; $J_{f}$ and $J_{f}$ are both parallel to the front surface of the target; $B^{S}$ is oriented along $+ \hat{z}$ ; the green-dotted line indicates the trajectory of the surface fast electrons; the heat maps in the top left and bottom right represent the incident laser field and the currents, respectively. (c) NCS process. The red and purple arrows represent the polarization of the laser and $γ$ photons, respectively; $k_{γ}$ is the $γ$ photon wave vector; the heat map represents the reflected laser field, which propagates from left to right. (d) NBW process. The blue and green arrows represent the polarization directions of positrons and electrons, respectively, and the blue- and green-dotted lines represent their respective trajectories; the approximation of the magnetic field $B^{'}$ in the positron rest frame being equal to $B^{S}$ is employed; the heat map represents $B^{S}$ .
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Figure 2
(a) Angle-resolved positron polarization $S_{+}$ and (b) distribution $\log_{10} (d N_{+} / d Ω)$ with respect to the polar angle $θ$ and the azimuth angle $ϕ$ , respectively. Here $d Ω = \sin θ d θ d ϕ$ , $θ = 0 °$ in $+ \hat{x}$ , and $ϕ = 0 °$ in $+ \hat{y}$ . (c) Positron density $\log_{10} (n_{+} / n_{c})$ . The gray box represents the initial position of the foil; $e_{+}^{down}$ represents positrons propagating downward through the target, while $e_{+}^{up}$ represents positrons propagating upward along the target surface. (d) Positron angle distribution $d N_{+} / d θ$ and polarization $S_{+}$ vs $θ$ at $ϕ_{1} = 176 °$ (solid lines) and $ϕ_{2} = 200 °$ (dotted lines). Here we collect positrons within a range of $Δ ϕ = 1 °$ . (e) Energy-resolved positron density $d N_{+} / d ϵ_{+}$ and polarization $S_{+}$ vs positron energy $ϵ_{+}$ at $α$ . The gray-dotted lines indicate the FWHM of positron energy; the red-dotted line represents the results of neglecting the radiative polarization effects of positrons. All above results are at simulation time $t = 50 T_{0}$ . (f) Positron yield $\log_{10} N_{+}$ and polarization $S_{+}$ vs $t$ at $α$ . $t_{f} = 37 T_{0}$ is the time of laser departure.
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Figure 3
(a) $B^{S} / B_{0}$ and projection of electron density $\log_{10} (n_{e}^{'})$ in the $\hat{x} − \hat{y}$ plane. The red bidirectional arrow represents the laser polarization direction. (b) Angle distribution of $γ$ photons $\log_{10} (d N_{γ} / d Ω)$ with respect to $θ$ and $ϕ$ . Positron production mainly occurs within the black circle region ( $10 ° < θ < 30 °$ , $0 ° < ϕ < 60 °$ ) [88]. (c) Projection of photon density $\log_{10} (n_{γ}^{'})$ for photon energy $ϵ_{γ} > 300 MeV$ in the $\hat{x} − \hat{y}$ plane. The green-dashed line represents the front surface of the target; the black and purple lines represent the contour lines of the average nonlinear QED parameter ${\bar{χ}}_{γ} = 0.35$ and ${\bar{χ}}_{γ} = 0.15$ , respectively. (d) Polarization-resolved distribution of $γ$ photons $d N_{γ} / d Θ$ vs $Θ$ . $B^{'} \approx ϵ_{+} [B^{S} − {\hat{v}}_{+} ({\hat{v}}_{+} \cdot B^{S})]$ and $B^{S} ∥ + \hat{z}$ are employed; the $γ$ photons originate from the peak cone angles ( $19 ° < θ < 21 °$ , $- 1 ° < ϕ < 1 °$ ) and ( $24 ° < θ < 26 °$ , $25 ° < ϕ < 27 °$ ) for the cases of $ϕ_{L} = 0 °$ (red dotted) and $ϕ_{L} = 45 °$ (blue solid), respectively. (e) $| {\bar{S}}_{+} |$ with respect to $Θ$ and $ϵ_{+} / ϵ_{γ}$ . The black arrows represent ${\bar{S}}_{+}$ ; the red line indicates the pair creation probability $d W / d (ϵ_{+} / ϵ_{γ})$ for $Θ = 90 °$ , normalized by its maximum; $χ_{γ} = 0.4$ is employed. (f) Projection of $B_{z}^{S} / B_{0}$ in the $\hat{x} − \hat{y}$ plane. The blue, green, and gray lines with arrows represent trajectories of the downward positrons $e_{+}^{down}$ , the newborn electrons, and the upward positrons $e_{+}^{up}$ , respectively; $l_{0}$ is the spatial dimension of $B^{S}$ . All of the above simulation results are at $t = 35 T_{0}$ .
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Figure 4
The yield $\log_{10} N_{+}$ and average polarization degree ${\bar{S}}_{+}$ of the downward positrons $e_{+}^{down}$ vs (a) the foil tilt angle $θ_{T}$ , (b) the laser polarization angle $ϕ_{L}$ , (c) the laser peak intensity $a_{0}$ , and (d) the preplasma scale length $L_{pre}$ , respectively. Other parameters are the same as those in Fig. 2.
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